The production of synthesis gas for Fischer-Tropsch synthesis is typically obtained by passing a hydrocarbon feed through primary and secondary reforming stages. The primary reforming is often conducted in tubular steam reformers or heat exchange reformers, while the secondary reforming is typically conducted in autothermal reformers. When combining a heat exchange reformer with a subsequent autothermal reformer, the hot effluent gas from the autothermal reformed is usually used as heating medium in the heat exchange reformer. It is known to recycle tail gas from Fischer-Tropsch synthesis as part of the hydrocarbon feed used to produce the synthesis gas in the primary and secondary reforming stages. Tail gas can be added prior to the primary reforming or to the primary reformed gas before entering the secondary reforming (typically autothermal reforming).
Tail gas from Fischer-Tropsch synthesis contains hydrogen, carbon monoxide, carbon dioxide as well as light hydrocarbons in the form of paraffins such as methane, ethane, propane and not least olefins such as propylene. The tail gas may also include alcohols and other higher hydrocarbons of both paraffinic and olefinic nature. It is known that the addition of such tail gas to the synthesis gas production section enables that there is sufficient carbon dioxide during the reforming to achieve the desired H2/CO molar ratio, typically about 2.0.
As used herein “tail gas” means off-gas from the Fischer-Tropsch synthesis stage which is not re-used in said stage.
Hydrogenation of tail gas is known in the art. For instance, in GB 632386 tail gas from Fischer-Tropsch synthesis is hydrogenated in order to increase the otherwise low heating value of this gas caused by the presence of i.a. carbon dioxide, carbon monoxide and hydrogen.
WO-A-0142175 discloses a process in which tail gas is hydrogenated in order to saturate any unsaturated hydrocarbons and is then reformed in a separate steam reformer. The hydrogenation serves to decrease the tendency towards coking in the subsequent high temperature treatment of the steam reformer, since the tendency to coking in said reformer is greater when unsaturated hydrocarbons are present in the tail gas. The resulting reformed tail gas may subsequently also be passed to an autothermal reformer. Accordingly, a steam reformer is used between the hydrogenation stage and the autothermal reformer.
EP-A-1860063 discloses a process in which off-gas from Fischer-Tropsch synthesis where olefins present in the off-gas are first hydrogenated and then converted to hydrogen by a reforming process. Olefins are hydrogenated because of carbon deposition or coking of catalysts used in the hydrogen manufacturing unit and which form hot spots on the catalyst and the reformer reactor tubes. Thus, olefins are removed to avoid coking in a steam reformer having reformer tubes such as a fired reformer.